The following points detail the importance of supplying oxygen to the wort just before fermentation:

Ever had a stuck fermentation? This may be due to low oxygen content in the wort which puts stress on the yeast and reduces their ability to reproduce.

Ever had a beer that tasted sweet? This is due to the yeast not completely metabolizing the malt sugars to the attenuation level expected. This can also be due to low oxygen content in the wort which places stress on the yeast.

Ever tasted a homebrewed beer that tasted funky? These are off flavors that can be created due to the yeast stress which can be a result of low oxygen content.

So what can be done to ensure that oxygen is present in the wort to adequate levels?

Oxygen content in the wort is dependent on the original (starting) gravity, yeast cells being pitched, and the desired quantity by the end of fermentation. As each of these stated factors increases the oxygen content also needs to increase.

Stirring a fermentor by hand by rolling it on the floor will not achieve more than 10 ppm oxygen content in your beer. This is a good number for most beers but it will require a vigorous stirring for a minimum of 10 minutes (although realistically it may take up to a half an hour). To quickly achieve oxygen content needed the best way is to use a sintered stone and an oxygen tank. This can reduce the time to 1 minute of purging to adequately get oxygen levels up to 10 ppm. An oxygen tank also allows you to exceed 10 ppm with further aeration time.

Some people use an aerator mixer at the end of a racking can and the truth is that it may help to increase the oxygen content slightly but it should not be the only method for aeration. If you are not using an oxygen tank, my recommendation would be to use the aerator at the end of the racking cane followed up with at least 10 minutes of good manual mixing rolling the carboy on the floor.

Too much oxygen (>20ppm) which is difficult to achieve may impart more fusel alcohols and increased acetaldehyde. This may impart a “hot” cidery apple flavor which would not be desired.

At the end of the day the important tip to remember is that the yeast need oxygen early in fermentation to allow growth and reproduction. This becomes even more important as the starting gravity increases where additional yeast pitch rates will also be required. This could be the one thing you are missing to turn your average beer into a great beer.